The present invention relates to a construction of a cylinder injection engine or a direct fuel injection engine with two or more spark plugs per each cylinder, a control apparatus and a control method therefor.
As shown in Japanese Patent Application Laid-Open No. JP-A-8-35429, a cavity is provided in a piston of the conventional cylinder injection engine to concentrate fuel within the cavity to make a mixture of air and fuel around a spark plug rich for certainly providing ignitionability even if the air/fuel ratio of the mixture in the overall cylinder is lean. However, since the mixture around the spark plug is rich, NOx tends to be generated. In particular, when a stratified charge combustion operation lasts in increasing of engine load, a problem is encountered in increasing of NOx emission. For purification of exhaust gas, it heavily depends on performance of NOx purifying catalytic converter. On the other hand, NOx can be reduced by EGR (Exhaust Gas Recirculation) to a homogeneous mixture. However, when single spark plug is employed, flame propagation velocity is lowered according to increasing of EGR amount to possibly make combustion unstable to degrade fuel saving effect. While a construction, in which two spark plugs are provided, has been put into practical use, improvement of fuel economy is still insufficient.
The present invention realizes quick combustion with dual-plug ignition system by applying EGR to a homogeneous mixture in a region where an engine output torque is relatively large by controlling stratified mixture and homogeneous mixture in the case where dual-plug ignition system is employed in a cylinder injection (direct fuel injection) system. In a range where the engine output torque is relatively small, stratified charge combustion operation is performed for improvement of fuel economy by super lean burn with large amount of EGR. In the later case, since the engine output torque is small and emission of NOx is small to be satisfactorily treated by post process after combustion. In a range where the engine output torque is further small, homogeneous charge compression ignition operation is performed. By this, reduction of both of fuel consumption and exhaust emission can be achieved.
An object of the present invention is to provide a construction of a cylinder ignition engine having two spark plugs in each cylinder, which enables homogeneous EGR operation and stratified EGR operation depending upon operating condition, fuel injection, air flow control, ignition control system and a control method therefor.
Another object of the present invention is to provide a control method for performing compression ignition without using the spark plug to realize low NOx emission.
According to one aspect of the present invention, a cylinder injection engine wherein each of cylinders is provided with a fuel injector for injecting fuel into a combustion chamber formed between a cylinder head and a piston, an intake port opening to the combustion chamber, an intake valve engaging with the intake port, a plurality of spark plugs provided for each cylinder for effecting ignition of a mixture formed within the combustion chamber, an external EGR valve having a function of EGR amount control to the cylinder, an intake and exhaust control valve, and means for setting air/fuel ratio modes and means for setting mode of switching spark plugs.
Preferably, the air/fuel ratio mode setting means has at least a stratified charge lean combustion mode concentrating the mixture around the spark plug, a stoichiometric mode for forming a homogeneous mixture and a homogeneous lean combustion mode.
Preferably, the air/fuel ratio mode setting means further has a compression ignition mode.
Preferably, two spark plugs are provided in each cylinder, a fuel is injected in an intake stroke, dual-plug ignition is performed under the condition where EGR is effected and, the mixture is concentrated to one of the spark plugs in stratified charge combustion operation.
Further preferably, when an engine revolution speed is lower than a predetermined value, the mixture is concentrated to the spark plug close to a tip end of the fuel injector, and when the engine revolution speed becomes higher than the predetermined value, the mixture is concentrated to the ignition valve located remote from the tip end of the fuel injector.
Still further preferably, the fuel injector is arranged on the side of suction stoke side of the cylinder, a tumble control valve is employed as a air flow control means for controlling the tumble control valve in closing direction under stratified charge combustion operation for strengthen tumble for concentrate the fuel injected in the compression stroke to one of the spark plugs by the tumble flow.
By the foregoing control, homogeneous mixture is formed in the range where the engine output torque is large to realize fast combustion upon application of EGR by dual-plug ignition. Furthermore, in the range where the engine output torque is smaller than the former mode, more fast combustion in stratified mixture with applying EGR is achieved to whereby achieve both of fuel economy and reduction of emission (NOx component).